The present invention relates to a telephoto lens system. More particularly, the present invention relates to a compact telephoto lens system having a high-performance 35 mm for use with a single-reflex lens camera, which lens is lightweight and of small size and which includes a lens group movable for achieving focusing.
Many types of inner focusing telephoto lens systems have been developed. An advantage of such lens systems is that focusing can be carried out with a small force and the user can hold the lens system securely while the lens system has a very small amount of movement in its center of gravity. An example of such a lens system is described in Unexamined Published Japanese patent application No. 214008/1984. Due to these advantages, inner focusing telephoto lens systems have gained growing importance in today's AF (auto focus) oriented camera market.
An important condition to be met by an AF adapted lens system is for the lens group which is moved during focusing to be lightweight and that focusing to be achievable without imposing an undue load on the AF drive unit. It is generally unavoidable for a telephoto lens system to employ optics of a large size, which makes the use of an inner AF design essential.
However, inner focusing lens systems, which require a certain amount of movement of a focusing group to be provided for within optics, inevitably have an increased overall length. If the refractive power of the focusing group is increased to decrease the amount of its movement, great aberrational variations occur when the system is focused for an object at a near distance. For these reasons, it has been difficult to attain a compact inner focusing lens system as compared with other types of telephoto lens system that achieve focusing by advancing either the whole system or the front lens group.